This invention relates to cast metal-matrix composite materials, and, more particularly, to such composites having a matrix alloy tailored to avoid the formation of harmful intermetallic phases.
Cast composite materials are formed by melting a matrix alloy in a reactor and then adding solid particulate matter. The mixture is vigorously mixed to encourage wetting of the matrix alloy to the particles, which remain solid during the mixing, and after a suitable mixing time the mixture is cast into molds or forms. The molten metallic matrix solidifies as it cools, resulting in a cast solid composite material. The mixing is conducted while minimizing the introduction of gas into the mixture.
The cast composite materials have fully wetted particles, few voids, and a generally uniformly mixed structure. Such cast composite materials are much less expensive to prepare than other types of metal-matrix composite materials such as those produced by powder metallurgical technology. Composite materials produced by this approach, as described in U.S. Pat. Nos. 4,759,995 and 4,786,467, have enjoyed commercial success in only a few years after their first introduction.
One potential application of cast composite materials is in foundry remelt alloys. The composite materials are prepared by a supplier and cast into ingots at the supplier's plant. The cast ingots are transported to a commercial foundry, where they are remelted and cast to the final shape required by the customer. This foundry remelt approach is commonplace throughout industry for the processing of conventional aluminum alloys, and the introduction of aluminum-based cast composite materials into many applications is practical only where they can conform to this approach.
Experience has shown that, with the proper mixing technique, a wide variety of cast composite materials can be mixed by the suppliers. In the mixing step, the maximum temperature to which the molten composite may be heated is normally limited to avoid the production of unwanted reaction products between the matrix alloying elements and the reinforcement particles. Some reaction products can reduce the mechanical properties of the composite material and cause porosity in the composite material, and are therefore to be avoided.
However, many of these cast composite materials are not compatible with commercial foundry remelt practices. Cast composite materials used in remelt applications must permit high remelt temperatures, typically greater than those used in the composite mixing operation, and long remelt holding times. The casting of metallic composite materials into complex shapes requires that the molten material be superheated above its melting point and be highly fluid so that it can flow into cold mold cavities for a considerable distance before the superheat is removed and the metal freezes. The greater is the remelt temperature permitted for the material and the fluidity of the material, the greater is the distance the molten composite material may flow into mold cavities before it solidifies, and the more intricate the products that can be cast.
Additionally, present foundry techniques usually call for the melting of large masses of the casting alloy to reach a stable temperature distribution, and casting articles from the large melted mass. The remelted material may remain at elevated temperature for extended periods of time, such as up to 24 hours, before casting. During this holding period, the castability of the composite material may degrade, so that a composite material may be much less castable after such a holding period then if cast immediately upon remelting. It is important that the composite material be castable by such commercial practices that have been long established, to accelerate the acceptance of the composite material by foundrymen.
In one specific example, aluminum-7 weight percent silicon alloys have been used in industry for years as remelt alloys, because the alloy has good fluidity and acceptable mechanical properties after casting. A satisfactory composite material of, for example, 15 volume percent of silicon carbide particles in an aluminum-7 weight percent silicon alloy may be prepared and cast by the supplier with a maximum temperature of 1265.degree. F. in the mixing process. Ingots of this alloy are furnished to a foundry remelter, who remelts the ingots and holds the molten composite at a conventional remelt temperature of about 1450.degree. F. for 8 hours before casting. The molten composite material casts very poorly, has low fluidity, and results in unacceptable product. The composite material is therefore rejected for the particular application, even though it might otherwise provide important benefits to the final product.
There therefore exists a need for an improved approach to the preparation of cast composite materials, particularly those for use in foundry remelt applications. The present invention fulfills this need, and further provides related advantages.